Fluid heaters



J. THURLEY FLUID HEATERS July 25, 1961 3 Sheets-Sheet 1 Filed Aug. 11,1958 J. THURLEY July 25, 1961 FLUID HEATERS 3 Sheets-Sheet 2 Filed Aug.11, 1958 QQJ F/GB.

July 25, 1961 J. THURLEY 2,993,479

FLUID HEATERS Filed Aug. 11, 1958 a SheetsSheet s F/ G. 4. F/ G. 5.

Q7 72 U3 4 .37 U2 k p I N 1 3 I 3 0 w 9 0 0W0 0 0 9 00000 000 0 0 0 0 a0 0 0 0 0 0 0 0 0 00 0000000v 0 0 0 0 0 0 s 0000000000v 9 P. Ir 3%57. 27 3@/5 7 I 7 I a q L@ 0000000000 0000000 00 W V 0 0 a 00000000 8 m000000 0 P v 0 00000000 00000 000m0 9 7 00000000 0 9 h 0 3 H 2 @1 0:32@B? United States Patent 2,993,479 FLUID HEATERS John Thurley,Westminster, London, England, assignor to Gibbons Heaters Limited,London, England, a

British Company Filed Aug. 11, 1958, Ser. No. 754,466 Claims priority,application Great Britain May 14, 1958 8 Claims. (Cl. 122-23) Thisinvention concerns improvements in or relating to fluid heaters in whichthe fluid to be heated is conveyed through tubes in a furnace and heatis transferred to this fluid by radiation and/or convection fromcombustion gases or flames in the furnace. More particularly, theinvention relates to heaters for heating temperature sensitive stocks,such as hydrocarbons.

In fluid heaters used heretofore, aluminous highly emissive flame hasbeen considered the most satisfactory means for heating the furnace dueto the ability of such a flame to radiate most heat for a given furnacetemperature. In the methods used for heating by radiation, the tubes,which usually extend either horizontally or vertically, of the furnace,are arranged in sets lining up posite Walls of the latter and aplurality of burners are arranged at each end of the furnace to directtheir flames between the sets of tubes. These tubes are arranged so asnot to come into direct contact with the hot combustion gases or flamesand a high flame emissivity is relied upon in order that as much heat aspossible should be transferred to the fluid being heated before thecombustion gases leave the furnace, e.g. to pass to a convection bank.

Such heaters suffer from the disadvantage that heat absorption is notuniform around the periphery or along the length of each tube and alsovaries from one tube to another depending on the position of each tuberelatively to the burners. In refinery heaters Where the fluid to beheated is a temperature sensitive stock liable to coke when overheated,the side of the tube which is nearer to the burner flames becomes heatedto a greater extent than that side remote from the burner flames andconsequently coking of the stock inside the tube is greater at the sidefacing the burner flames, the resultant build up of carbon at this sideresulting in high metal skin temperatures of the tube and ultimatefailure of the latter. Further, in light hydrocarbon pyrolysis andcatalytic reforming, high heat absorption rates and high temperaturesare required at a high temperature level. Such high heat absorptionrates and the uneven tube temperatures produced cause scaling of theouter surface of the tubes and ultimate failure of the latter.

Many attempts have been made to solve the problem of how to ensure evenheat absorption around and along the tubes, but none of these attemptshas proved entirely satisfactory.

In one heater, burner flames are made to impinge and flow downrefractory walls located on each side of a double row of staggeredtubes. In such a heater more even heating of the tubes is achieved and ahigher average rate of heat absorption is permitted than in the case ofconventional heaters. However, in order that the said refractory wallsmay form a radiant heat source they must be subjected to severe flameimpingement and as a result become damaged in use, especially whenoitfired burners are used.

In a second heater proposed in an endeavour to overcome the disadvantageof uneven heating of the tubes, a stream of waste combustion gases isblown at high speed between the burner flames and the tubes being heatedin order to form abarrier preventing flames licking the tubes.- Thevelocity of these gases may be varied to control the height of the saidbarrier and thus the amount of heat absorbed by the tubes. This heater,besides not being perfectly effective in use, is more elaborate than theconventional form of heater and requires a fan for the recirculation ofthe waste gases.

In a third heater, which comprises a cylindrical furnace in whichvertical tubes are arranged around the inner periphery of the furnace, ahighly radiating luminous flame burner is arranged at the lower part ofthe furnace and, in order to compensate for the loss of flame radiationat the upper part of the furnace, a cone is arranged in such upper part,such cone increasing the velocity of the flue gases, increasing theconvective heat transfer and also acting as a source of secondaryradiation. This arrangement gives a constant heat flux along the wholelength of each tube, but does not produce a constant heat flux aroundthe tube periphery since the sides of the tubes remote from the centreof the furnace absorb less heat than those sides facing the centre ofthe furnace.

In a further heater designed in an endeavour to ensure equal absorptionaround the tubes, 9. special form of tile is located over the uppertubes so as to ensure similar gas flow over these tubes and also tocompensate the high radiant heat absorption from the flames on the lowersides of the tubes by convective heat transfer to the upper sides of thetubes. This form of heater does not in fact provide sufficientcompensation and uneven heat absorption persists.

Further attempts to equalize the heating of the tubes have been made byplacing on each side of a single row or a double row of staggered tubesradiant panels, which may be the furnace walls, or radiant cups heatedby flameless combustion multi-burner installations. These attempts havebeen to some extent successful, but the heaters incorporating radiantpanels or cups are extremely expensive.

In all of the heaters discussed above, an attempt has been made toachieve maximum emissivity, either by using high emissivity flameburners or, if flameless burners are used, by using the burners to heatrefractory panels or cups in order to create a highly emissive heatsource.

It is an object of this invention to provide a new or improved fluidheater, particularly a refinery heater for hydrocarbons, in which even,or substantially even heating, of the tubes is achieved without the needfor the use of extremely expensive equipment.

This object is achieved by using, for the or each burner, a short flamehigh intensity combustion burner capable of delivering a high velocitystream of combustion gases with a low emissivity and directing the oreach stream of combustion gases into a furnace space, through which passtubes carrying the fluid to be heated, from a position adjacent a wallof the furnace space towards an opposite wall thereof.

We have found that, contrary to expectations, the low emissivityradiation produced by a short flame high intensity combustion burnerproduces perfectly satisfactory heating of the tubes without radiatingpanels or cups being required and that the high velocity of thecombustion gases of such burner can be used to cause automaticcirculation of the combustion gases throughout the furnace without theneed for using circulating fans and even, or substantially even, heatingof the tubes is achieved.

In addition, the following further advantages are given by a heateraccording to this invention:

The heater may be constructed to be free from air infiltration, thecombustion gases issuing at a high velocity from high intensitycombustion burners occupy the whole of the furnace space and prevent airinfiltration, which would otherwise reduce the furnace temperature,reduce the percentage of the important radiating gases CO and H O in thecombustion gases and reduce the mean beam length of the radiating massof combustion gases.

The heater may be made more compact since in high intensity combustionburners nearly complete combustion takes place inside a comparativelysmall refractory lined combustion chamber in the burner itself atcombustion rates of up to 10,000,000 B.t.u./hr. for every cubic foot ofcombustion space and the furnace does not therefore require to act as acombustion space and the tubes carrying the fluid do not have to bespaced remote from the burners to avoid flame impingement.

The excellent mixing of fuel and air and the low heat loss duringcombustion in high intensity combustion burners enables substantiallycomplete combustion of the fuel to be achieved without smoking, whilstusing substantially no excess air above that required for burning of thefuel. This produces a higher flame temperature and a The tubes 7 are, asshown, arranged in parallel rows.

In one end wall 3 of the furnace 1 are mounted a plurality of shortflame high intensity combustion bumers 8, each capable of producing ahigh velocity jet of combustion gases with a low emissivity. The burners8 a are arranged spaced one above the other in a vertical row i 4 whichis centrally located with respect to the rows of higher CO and H 0content in the combustion gases of the tubes 7, the arrangement beingsuch that the jets of combustion gases from the burners travelhorizontally between the rows of tubes 7 and impinge on the end wall 3of the furnace 1 opposite to that end wall 3 in i which the burners aremounted.

high temperature superheaters which arise due to low velocity combustiongas streams in which the gases are fusion temperature ashes includingsodium sulphate.

Moreover, the operation of the. burners without excess air and theelimination of infiltrated air to the, furnace prevents the oxidationand scaling, and thereby prolongs the life, of intermediate tubesupports andof the tubes in high temperature service.

The jet of combustion gases issuing from a high in:

tensity combustion burner is directional and thus the ori:

entations of a plurality of burners can be arranged to provide theoptimum combustion gas circulation pattern in the furnace.

-High intensity combustion burners may be used which utilise refineryfuel gas or residual fuel oil which is now tending to replace refineryfuel gas as a refinery heating fuel. When using refinery fuel gas, lowemissivity. comlow emissivity combustion gases at low velocity may be Iproduced with premix gas burners using a fuel gas pressure above 25p.s.i.g. at the limiting orifice in, order to aspirate the primary air.7 I v The furnace may be designed to be long and narrow, thereby givingoptimum radiant absorption characteris tics and enabling the use oflonger tubeswhich reduce the capital cost of the heater and reduce theprocess pressure drop.

In order that this invention may more readily be understood, referencewill now be made by way of example to the accompanying drawings inwhich: FIGURE 1 is a diagrammatic perspective view of a heater accordingto this invention, part of this "heater being broken away to show theinterior of the furnace; 7

FIGURES 4 to 7 are diagrammatic horizontal sections through furtherheaters according to this invention.

Referring to FIGURES l and 2, the heater there-illus trated comprises afurnace 1 of box type construction and including a furnace space 2defined by walls 3.: The walls 3 are formed of a plurality ofinterconnected panels and are carried by a supporting frame of girders.4 mounted on base supports 5. At its upper part thesaid {I have foundthat, using high velocity jets of combustion gases of low emissivity asdescribed above, eificient circulation of these gases occurs, asindicated by the arrows in FIGURE 2, and perfectly satisfactory evenheating of the tubes 7 is obtained.

Circulation of the combustion gases in the furnace space 3 may also beachieved if, instead of using burners which produce high velocitycombustion gas streams in the form of jets, burners are used whichproduce high in addition swirled within the streams. Swirling of thegases produces a region of low pressure in the centre of the furnacespace and thereby cooler combustion gases at the rear and sides of thetubes are drawn towards the furnace centre and this circulation of gasesgives a more even furnace temperature with a more even heat flux aroundthe periphery and along the length of each tube than is obtained withconventional heater designs.

In a modification illustrated in FIGURE 3, each burner 8 is fired into amuflle 9 which comprises a short cylindrical tube spaced from thefurnace wall 3 in which the burner is mounted and surrounding thedischarge end of the burner so as to be coaxial therewith. Due to theaspiration effect of this muffle 9, combustion gases in the furnacespace 3 are drawn through the muflle and are entrained with the jet ofcombustion gases issuing from the burner so that increased circulationof the combustion gases is achieved. When a muffle is used, thecombustion gases drawn through the mufile temper the high burner flametemperature before the gases leave the muffle. The circulation ofcombustion gases in the furnace not only creates even furnace conditionsby lowering the temperature at the centre of the furnace and raising thefurnace temperatures remote from the burners, but also increases heatabsorption by convective as well as radiant heating. Although thefurnace of the heater of FIGURE 1 has been shown as of box typeconstruction, it will be appreciated that this furnace could be ofcircular or any other suitable form. Further, although the tubes 7 havebeen shown as being vertically arranged and the burners as firinghorizontally, it will be appreciated that the tubes could be arrangedhorizontally or in any other desired orientation and the burners couldbe arranged to fire vertically, e.g. the heater could be floor-fired, orin any other desired orientation. Thus, FIGURE 2 could equallywellrepresent a heater in which the tubes are horizontally arranged andthe burners are mounted in the floor of the furnace to fire verticallyupwardly.

According to a further feature of the invention, the combustion gasesmay be removed from the furnace at the end opposite that from which theburner streams are directed and recirculated through the furnace bybeing brought externally of the latter and reintroduced into the furnaceso as to be entrained with the coma bustion gases issuing from theburners. Advantageously, such external recirculation is achieved byfiring. the burners, which are located externally of the furnace, into amuflle mounted in, or forming part of, the furnace wall.

FIGURES 4 to 7 show diagrammatically heaters in which the combustiongases are externally recirculated. These heaters are shown as beingfloor-fired and have extensions 10 at their upper parts which lead toflue stacks or waste heat recovery sections. Due to their compactness,several of these radiant heaters could be combined and the flue gasesled off to a common process convection bank or waste heat recovery unitor stack. The burners 8 in each of these heaters are located externallyof the furnace 1 and the mutfles 9 are mounted in the floor or lowerwall 3 of the furnace. From apertures 11 in the extensions 10 conduitmeans, indicated schematically by the lines 12, lead to the muflles 9 sothat combustion gases in the furnace space 2 are automatically drawnfrom the upper part of the furnace space, brought externally of thelatter to the muflies 9 and entrained with the combustion gases issuingfrom the burners.

The high velocity streams of the burners used in heaters according tothis invention enable external recirculation of the combustion gases tobe effected without circulating fans being required.

Conveniently a damper 13 is provided in the external flow path of therecirculating combustion gases so as to afford control of the furnaceatmosphere, i.e. control of the ratio of convection heat absorption toradiant heat absorption.

FIGURES 4 to 7 show heaters having various arrangements of tubes 7. Theheater of FIGURE 4 has two horizontally spaced vertical rows of tubes,whilst the heaters of FIGURES 5 and 6 have respectively three and fivesuch rows. The heater of FIGURE 7 has nine rows, the tubes of each rowbeing vertically staggered relatively to the adjacent rows.

Those tubes 7 which are nearest in the line of the combustion gasesissuing from the burners may be omitted as in the heater of FIGURE 6 or,alternatively, may, as in the heater of FIGURES, be shielded from directimpingement of the high velocity gas streams by baffles 14.

The spacings of the tubes 7 may be small or large depending on whetherone requires minimum furnace size or maximum coil heat absorption. In aconvenient form of the heater of FIGURE 1, the height of the furnacespace 2 may be 40 feet and the rows of tubes 7 spaced apart by adistance of 3 feet.

Further, the heater according to this invention may be used to heat anygaseous or liquid medium and the invention may thus be applied to asteam boiler. It is, however, as previously indicated, particularlyapplicable to refinery heaters.

Examples of high intensity combustion burners which utilise residualfuel oil and produce a high velocity jet of combustion gases and whichare suitable for heaters according to this invention are shown anddescribed in US. Patents Nos. 2,625,795, 2,632,300 and 2,701,608, whilstexamples of high intensity combustion burners which produce a highvelocity combustion gas stream in which the gas is caused to swirlwithin the stream are shown and described in US. Patents Nos. 1,560,076,1,560,078 and 2,698,050.

I claim:

1. A heater of the class described comprising: Wall structure defining afurnace space; a plurality of tubes passing through said furnace spaceand connected to be supplied with fluid to be heated, said tubes beingspaced from said wall structure and from each other to allow the freeflow of gas around the tubes; a plurality of short flame, high intensitycombustion burner means having combustion chambers in whichsubstantially complete fuel combustion takes place, said combustionchambers sivity to cause such jets to impinge on a part of said wallstructure remote from the burner means orifices andto cause circulationof such gases around the tubes in the furnace space, said jets issuingfrom said burner means orifices being substantially the sole means transferring heat to said tubes.

2. A heater of the class described comprising: wall structure defining afurnace space; a plurality of tubes passing through said furnace spaceand connected to be supplied with fluid to be heated, said tubes beingspaced from said wall structure and. from each other to allow the freeflow of gas around the tubes; a plurality of short flame, high intensitycombustion burner means having combustion chambers in whichsubstantially complete fuel combustion takes place, said combustionchambers being substantially entirely shielded from said furnace spaceandv positioned adjacent one surface of said wall structure definingsaid furnace space, said burner means each having a delivery orifice atsaid one surface of said wall structure for delivering to said furnacespace a high velocity jet of combustion gases having a low emissivity tocause such jets to impinge on a part of said wall structure remote fromthe burner means orifices and to cause circulation of such gases aroundthe tubes in the furnace space, said jets issuing from said burner meansorifices being substantially the sole means transferring heat to saidtubes; and an axially short cylindrical muffle mounted adjacent eachburner means and positioned relatively to said wall structure so thatthe combustion gas jet from each burner means orifice passes coaxiallythrough its respective muffle before passing across the burner space anddraws therethrough combustion gases circulating in the furnace spacethereby to increase the circulation of these gases.

3. A heater of the class described comprising: wall structure defining afurnace space; a plurality of tubes passing through said furnace spaceand connected to be supplied with fluid to be heated, said tubes beingspaced from said wall structure and from each other to allow the freeflow of gas around the tubes; a plurality of short flame, high intensitycombustion burner means having combustion chambers in whichsubstantially complete fuel combustion takes place, said combustionchambers being substantially entirely shielded from said furnace spaceand positioned adjacent one surface of said wall structure defining saidfurnace space, said burner means each having a delivery orifice at saidone surface of said wall structure for delivering to said furnace spacea high velocity jet of combustion gases having a low emissivity to causesuch jets to impinge on a part of said wall structure remote from theburner means orifices and to cause circulation of such gases around thetubes in the furnace space, said jets issuing from said burner meansorifices being substantially the sole means transferring heat to saidtubes; and an axially short cylindrical muffle mounted adjacent eachburner means and positioned relatively to said wall structure so thatthe combustion gas jet from each burner means orifice passes coaxiallythrough its respective mufile before passing across the burner space anddraws therethrough combustion gases circulating in the furnace spacethereby to increase the circulation of these gases and conduit meansleading from a region of the furnace space remote from the burner meansto said muflles so that combustion gases in the furnace space arewithdrawn therefrom and entrain with combustion gas jets passing throughsaid muflies.

4. A heater according to claim 3, wherein damper 7 means are provided insaid conduit means ror controlling the atmosphere of the furnace space.

5. A heater according to claim 3, wherein baffles are positioned thefurnace space for shielding the tubes closest to the burner meansorifices from direct impingement of the combustion gas jets.

6. A fluid heater of the class described comprising: walls defining afurnace space; a plurality of tubes passing through said furnace spaceand connected to be supplied with the fluid to be heated, said tubesbeing arranged parallel to one another in a plurality of spaced rows,the outer rows of tubes being spaced from opposed 'walls of the furnacespace and the tubes in each row being spaced from each other to allowthe free flow of gas around the tubes; a plurality of short flame, highintensity combustion burner means, each burner means having a combustionsection in which substantially complete combustion of fuel takes placeand which delivers from a delivery orifice thereof a high velocitystream of combustion gases having a low emissivity, said combustionspace being substantially entirely shielded from said furnace space,means mounting the burner means adjacent said furnace space; deliveryorifices of the combustion sections of said burner means communicatingthrough one of said walls with said furnace space so as to direct thecombustion gases from said burner means into the furnace space andbetween the said rows of tubes, whereby said jets issuing from saidburner means orifices constitute substantially the sole meanstransferring heat to said tubes, said orifices being closely adjacentsaid one wall and nearer to the latter than any of said tubes; and aplurality of axially short tubular mufiies, one associated with each ofthe burner means, each mufiie being mounted within the furnace space andadjacent, but slightly spaced from/the said one wall so as to surroundthe delivery orifice of its associated burner means coaxially of thecombustion gas stream to issue therefrom.

7. In a fluid heater of the class described: walls defining -a chamber;a row of spaced apart parallel tubes for conducting fluid to be heatedthrough said chamber, each tube extending in direction between twoopposed walls of the furnace chamber and said row extending between afiring wall and a back wall; means for heating said tubes substantiallyuniformly throughout the respective exterior surface areas of each ofthem comprising burner means for'introducing into said chamber heated,low emissivity gases at high velocity from a location between said rowof tubes and one of said opposed wall portions, said gases entering saidchamber substantially at the interior surface of said firing wall indirection towards said back wall; whereby said gases are circulatedbetween said row of tubes and said opposed walls and between all of saidtubes to transfer heat thereto, said gases thus constitutingsubstantially the sole means transferring heat to said tubes.

8. In a fluid heater of the class described: walls defining a chamber; aplurality of rows of spaced apart parallel tubes for conducting fluid tobe heated through said chamber, each tube extending in direction betweentwo opposed walls of the furnace chamber and said rows extending betweena firing wall and a back wall; means for heating said tubessubstantially uniformly throughout the respective exterior surface areasof each of them comprising burner means for introducing into saidchamber heated, low emissivity gases at high velocity from locationsbetween said rows of tubes and one of said opposed wall portions, saidgases entering said chamber substantially at the interior surface ofsaid firing wall in direction towards said back wall, whereby said gasesare circulated between said rows of tubes and said opposed walls andbetween all of said tubes to transfer heat thereto, said gases thusconstituting substantially the sole means transferring heat to saidtubes.

References Cited in the file of this patent UNITED STATES PATENTS

